Summer disease update

 Six extended wetting events since Jul 22 have dramatically increased summer disease pressures in the Winchester area: Jul 22: 6 hr wetting at 73° with 0.9 in. rain, Jul 23-24: 13 hr wetting at 73° with 0.51 in. rain, Jul 30-31: 21 hr wetting at 70° with 1.01 in. rain; Aug 1-2: 15 hr wetting at 72° with 0.45 in. rain, Aug 3-4: 19 hr wetting at 68° with 0.92 in. rain; and Aug 6: 6 hr wetting at 68° with 0.2 in. rain. Suffice it to say that Winchester's extended dry conditions of July have passed! Wetting events at these temperatures have favored development of sooty blotch and flyspeck and the rot fungi. Evidence of early season (scab), mid-season (Brooks spot), and late season diseases (sooty blotch and developing rots) is shown below:

Scab, sooty blotch and rot spots on unprotected Granny Smith fruit at Virginia Tech AREC, Winchester, VA, Aug 10, 2020.

Infection by Brooks spot (8 o'clock and 1 o'clock positions from the calyx), as well as sooty blotch and rot spots on unprotected Idared fruit, Aug 10, 2020. Infection by Brooks spot, one of the earliest "summer diseases", typically occurs about a month after petal fall.

Recent disease pressure has been somewhat heavier than Winchester in other areas of Virginia, especially east of the Blue Ridge. Amounts of rainfall from extended wetting events have varied, especially with the remnants of the recent hurricane. Most were not excessive in areas where rain was needed; some extended wetting occurred with only a few hundredths of an inch of rainfall. All of these have been favorable for rot development where protection was inadequate and volume of rainfall can impact fungicide residual. 

Fire blight-killed shoots harbor the rot fungi and are a common pre-harvest inoculum source. Due to cool temperatures throughout bloom, the Winchester area mostly escaped fire blight this year, but infection was more common east of the Blue Ridge and in the Roanoke area.

Accumulated wetting hours through the summer can be an indicator of summer disease pressure, particularly to predict the development of the sooty blotch and flyspeck (SBFS) fungal complex, but also can be a general indicator of rot pressure. To predict SBFS, we record accumulated wetting hours (ACW) from rainfall, fog, or dew, starting 10 days after petal fall. The action threshold of 250 ACW predicts that the SBFS fungi are present on unprotected fruit when the threshold is reached, and symptoms will soon appear with further incubation. Winchester passed the 250-hr threshold on July 21, and symptoms are now present on unprotected fruit, as shown above, with symptoms heavier at lower elevations, which have more wetting hours. Here is a listing of selected petal fall dates and current total wetting hour accumulation (ACW) for selected locations as of Aug 10: Winchester, May 5, 393 ACW; Staunton, May 5, 823 ACW; Roanoke, Apr 18, 447 ACW; Floyd, Apr 28, 470 ACW; Manassas, Apr 28, 694 ACW; Sperryville, Apr 28, 387 ACW; Crozet, Apr 18, 423 ACW; Carter Mountain, Apr 18, 558 ACW; Red Hill, Apr 18, 483 ACW; Lynchburg, Apr 13, 647 ACW; Rustburg, Apr 13, 573 ACW; Danville, Apr 13, 832 ACW. 

Unless unusual fruit disease-related events occur, this will be the final post for this year. With approaching harvest, carefully observe pre-harvest intervals and per acre per year restrictions for all fungicides and all crop protection products.

Bitter rot observed at Winchester; Sooty blotch and flyspeck thresholds reached at Winchester and Tyro, VA.

For predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we began wetting hour accumulation at Winchester from May 14. As of Monday, July 4, the 250-hr accumulated wetting hour threshold (ACW) has been surpassed at all of the wetness sensors at our AREC and at Tyro, VA.

At Winchester ACW: at 909 ft elevation, 442 hr (with the 250-hr threshold reached June 5); at 952 ft elevation, 341 hr (near the threshold); and at the 983 ft elevation, 280 hr ACW. At Tyro, VA we started wetting hour accumulation May 4, and as of July 4, a sensor placed at the 941 ft elevation had accumulated 426 hr (with the 250-hr threshold reached June 6); the one at 1165 ft. elevation had accumulated 264 wetting hours, and the one 1465 ft. elevation had accumulated 280 wetting hours. 

At Winchester in the past two weeks we had three significant extended wetting events favorable for SBFS and fruit rot development: June 23: 9 hr at 66°, with 0.55 in. rain, and June 27-28: 14 hr at 80-72° with 0.08 in. rain, and July 3-4: 17 hr at 68° with 0.02 in. rain. Bitter rot was observed in Honeycrisp apples at Winchester as early as June 22. Specific treatment against the SBFS fungal complex and rot diseases is advised.

Summer disease management should be directed toward the array of potential problems in specific orchard blocks, utilizing protective fungicides for general purposes, but supplementing them with more specifically active fungicides directed at higher disease pressures and special control problems. Expect higher fruit rot pressures where fire blight killed shoots and fruit mummies are present in the trees. For more information on the effectiveness of apple fungicides for management of sooty blotch/flyspeck, Brooks fruit spot, black rot, white rot and bitter rot consult Table 6 (pp. 53-54) in the 2016 Spray Bulletin for Commercial Tree Fruit Growers.

More summer disease weather

Summer disease pressure remained high this week with several nights of heavy dews and with 14 hours of extended wetting with an inch of rain at 69° Aug 9-10.

I was away for a week, and on my return some changes were obvious- lots of apple trees with light green, new growth, and nice red color on ripening (and tasty!) Gala apples.

Where powdery mildew is a problem, the new growth will be susceptible to mildew infection, leading to more overwintering. In such situations, the fungicides Indar and Inspire Super offer some suppression of mildew and other summer diseases such as sooty blotch and flyspeck. Both Indar and Inspire Super have restrictions preventing their application closer than 14 days to harvest. We suggest including a general protectant fungicide such as captan or ziram with the above fungicides to broaden the spectrum and residual activity.

Yesterday I saw striking examples of the effects of fire blight strikes on the prevalence of bitter rot in Nittany apples (shown below). The bitter rot fungus can colonize the dead twigs within six weeks of the initial fire blight blossom infection and become a bitter rot inoculum source for the rest of the year. Any fire blight strikes in a tree should be taken as a warning of a potential rot problem. Notice that the fruit immediately below also has sooty blotch, and that indicates that protective fungicide is lacking which would also permit more rot problems.

Lee's left-overs

Tropical storm Lee brought us rainfall that totaled 2.72 inches over several days, Sept. 4-7. The ground seemed to soak up this volume of rain surprisingly well, and it will be beneficial for sizing the apple crop. Of course this amount of rain and the extended wetting should be expected to have some consequences for disease development, particularly if fungicide residues were low. Keep watching for sooty blotch and flyspeck, as well for as fruit rots such as bitter rot.

For the record, we had extended wetting periods of 16 hr Sept 4-5 at 69º F, 47 hr Sept 5-7 at 69º F, 12 hr at 64º F Sept 7-8 with Lee, and a more recent one of 22 hr at 68º degrees. Generally, wetting in the higher temperature ranges favor more rapid rot infection and development, and ripening fruit are more susceptible to infection. Typically, residues of a protectant fungicide such as captan do not weather through more than two inches of rainfall in a one week period.

Summer disease update

Over the weekend we got some much needed rain, totaling 1.8 inches for June 18-20. We had two extended wetting periods: 17 hr at 72 F June 18-19 and 12 hr at 70 F June 19-20. Last week we accumlated 40 hours of wetting, mostly from rain. This brings us to a total of 228 hours toward the 250-hour action threshold for sooty blotch and flyspeck.

Accumulated wetting hours

Last week we had 34 hours wet, mostly from dew. This gives us a total of 188 hours toward the 250-hour threshold.

Rappahannock County (Washington, VA) had 209 wetting hours as of June 8, and probably will reach the 250-hr threshold for development of sooty blotch and flyspeck on unprotected fruit this week.

Summer disease outlook

Accumulated wetting hours (starting April 29, 10 days after petal fall, Apr 19): 165 hr. The threshold for presence of sooty blotch and flyspeck fungi on unprotected fruit is 250 hr. As of May 25 a hygrothermograph in Rappahannock county (Washington, VA) had logged 163 wetting hours, also using the Apr 19 petal fall and Apr 29 start accumulation date.

For practical purposes, cedar-apple rust galls near our AREC have completed their sporulation for this year. Rust lesions are still showing up on apple leaves that were not protected two weeks ago.